diff --git a/scrape/scrape.go b/scrape/scrape.go
index e9d172a3e..10e9119a2 100644
--- a/scrape/scrape.go
+++ b/scrape/scrape.go
@@ -268,6 +268,7 @@ type scrapeLoopOptions struct {
 
 const maxAheadTime = 10 * time.Minute
 
+// returning an empty label set is interpreted as "drop"
 type labelsMutator func(labels.Labels) labels.Labels
 
 func newScrapePool(cfg *config.ScrapeConfig, app storage.Appendable, jitterSeed uint64, logger log.Logger, options *Options) (*scrapePool, error) {
@@ -498,9 +499,9 @@ func (sp *scrapePool) Sync(tgs []*targetgroup.Group) {
 		}
 		targetSyncFailed.WithLabelValues(sp.config.JobName).Add(float64(len(failures)))
 		for _, t := range targets {
-			if t.Labels().Len() > 0 {
+			if !t.Labels().IsEmpty() {
 				all = append(all, t)
-			} else if t.DiscoveredLabels().Len() > 0 {
+			} else if !t.DiscoveredLabels().IsEmpty() {
 				sp.droppedTargets = append(sp.droppedTargets, t)
 			}
 		}
@@ -634,7 +635,7 @@ func verifyLabelLimits(lset labels.Labels, limits *labelLimits) error {
 
 	met := lset.Get(labels.MetricName)
 	if limits.labelLimit > 0 {
-		nbLabels := len(lset)
+		nbLabels := lset.Len()
 		if nbLabels > int(limits.labelLimit) {
 			return fmt.Errorf("label_limit exceeded (metric: %.50s, number of labels: %d, limit: %d)", met, nbLabels, limits.labelLimit)
 		}
@@ -644,7 +645,7 @@ func verifyLabelLimits(lset labels.Labels, limits *labelLimits) error {
 		return nil
 	}
 
-	for _, l := range lset {
+	return lset.Validate(func(l labels.Label) error {
 		if limits.labelNameLengthLimit > 0 {
 			nameLength := len(l.Name)
 			if nameLength > int(limits.labelNameLengthLimit) {
@@ -658,8 +659,8 @@ func verifyLabelLimits(lset labels.Labels, limits *labelLimits) error {
 				return fmt.Errorf("label_value_length_limit exceeded (metric: %.50s, label name: %.50s, value: %.50q, length: %d, limit: %d)", met, l.Name, l.Value, valueLength, limits.labelValueLengthLimit)
 			}
 		}
-	}
-	return nil
+		return nil
+	})
 }
 
 func mutateSampleLabels(lset labels.Labels, target *Target, honor bool, rc []*relabel.Config) labels.Labels {
@@ -667,37 +668,37 @@ func mutateSampleLabels(lset labels.Labels, target *Target, honor bool, rc []*re
 	targetLabels := target.Labels()
 
 	if honor {
-		for _, l := range targetLabels {
+		targetLabels.Range(func(l labels.Label) {
 			if !lset.Has(l.Name) {
 				lb.Set(l.Name, l.Value)
 			}
-		}
+		})
 	} else {
-		var conflictingExposedLabels labels.Labels
-		for _, l := range targetLabels {
+		var conflictingExposedLabels []labels.Label
+		targetLabels.Range(func(l labels.Label) {
 			existingValue := lset.Get(l.Name)
 			if existingValue != "" {
 				conflictingExposedLabels = append(conflictingExposedLabels, labels.Label{Name: l.Name, Value: existingValue})
 			}
 			// It is now safe to set the target label.
 			lb.Set(l.Name, l.Value)
-		}
+		})
 
 		if len(conflictingExposedLabels) > 0 {
 			resolveConflictingExposedLabels(lb, lset, targetLabels, conflictingExposedLabels)
 		}
 	}
 
-	res := lb.Labels(nil)
+	res := lb.Labels(labels.EmptyLabels())
 
 	if len(rc) > 0 {
-		res = relabel.Process(res, rc...)
+		res, _ = relabel.Process(res, rc...)
 	}
 
 	return res
 }
 
-func resolveConflictingExposedLabels(lb *labels.Builder, exposedLabels, targetLabels, conflictingExposedLabels labels.Labels) {
+func resolveConflictingExposedLabels(lb *labels.Builder, exposedLabels, targetLabels labels.Labels, conflictingExposedLabels []labels.Label) {
 	sort.SliceStable(conflictingExposedLabels, func(i, j int) bool {
 		return len(conflictingExposedLabels[i].Name) < len(conflictingExposedLabels[j].Name)
 	})
@@ -708,7 +709,7 @@ func resolveConflictingExposedLabels(lb *labels.Builder, exposedLabels, targetLa
 			newName = model.ExportedLabelPrefix + newName
 			if !exposedLabels.Has(newName) &&
 				!targetLabels.Has(newName) &&
-				!conflictingExposedLabels[:i].Has(newName) {
+				!labelSliceHas(conflictingExposedLabels[:i], newName) {
 				conflictingExposedLabels[i].Name = newName
 				break
 			}
@@ -720,15 +721,24 @@ func resolveConflictingExposedLabels(lb *labels.Builder, exposedLabels, targetLa
 	}
 }
 
+func labelSliceHas(lbls []labels.Label, name string) bool {
+	for _, l := range lbls {
+		if l.Name == name {
+			return true
+		}
+	}
+	return false
+}
+
 func mutateReportSampleLabels(lset labels.Labels, target *Target) labels.Labels {
 	lb := labels.NewBuilder(lset)
 
-	for _, l := range target.Labels() {
+	target.Labels().Range(func(l labels.Label) {
 		lb.Set(model.ExportedLabelPrefix+l.Name, lset.Get(l.Name))
 		lb.Set(l.Name, l.Value)
-	}
+	})
 
-	return lb.Labels(nil)
+	return lb.Labels(labels.EmptyLabels())
 }
 
 // appender returns an appender for ingested samples from the target.
@@ -1599,8 +1609,8 @@ loop:
 			// and relabeling and store the final label set.
 			lset = sl.sampleMutator(lset)
 
-			// The label set may be set to nil to indicate dropping.
-			if lset == nil {
+			// The label set may be set to empty to indicate dropping.
+			if lset.IsEmpty() {
 				sl.cache.addDropped(mets)
 				continue
 			}
@@ -1857,12 +1867,10 @@ func (sl *scrapeLoop) addReportSample(app storage.Appender, s string, t int64, v
 		ref = ce.ref
 		lset = ce.lset
 	} else {
-		lset = labels.Labels{
-			// The constants are suffixed with the invalid \xff unicode rune to avoid collisions
-			// with scraped metrics in the cache.
-			// We have to drop it when building the actual metric.
-			labels.Label{Name: labels.MetricName, Value: s[:len(s)-1]},
-		}
+		// The constants are suffixed with the invalid \xff unicode rune to avoid collisions
+		// with scraped metrics in the cache.
+		// We have to drop it when building the actual metric.
+		lset = labels.FromStrings(labels.MetricName, s[:len(s)-1])
 		lset = sl.reportSampleMutator(lset)
 	}
 
diff --git a/scrape/target.go b/scrape/target.go
index f54623954..3f3afbd3a 100644
--- a/scrape/target.go
+++ b/scrape/target.go
@@ -172,22 +172,20 @@ func (t *Target) offset(interval time.Duration, jitterSeed uint64) time.Duration
 
 // Labels returns a copy of the set of all public labels of the target.
 func (t *Target) Labels() labels.Labels {
-	lset := make(labels.Labels, 0, len(t.labels))
-	for _, l := range t.labels {
+	b := labels.NewScratchBuilder(t.labels.Len())
+	t.labels.Range(func(l labels.Label) {
 		if !strings.HasPrefix(l.Name, model.ReservedLabelPrefix) {
-			lset = append(lset, l)
+			b.Add(l.Name, l.Value)
 		}
-	}
-	return lset
+	})
+	return b.Labels()
 }
 
 // DiscoveredLabels returns a copy of the target's labels before any processing.
 func (t *Target) DiscoveredLabels() labels.Labels {
 	t.mtx.Lock()
 	defer t.mtx.Unlock()
-	lset := make(labels.Labels, len(t.discoveredLabels))
-	copy(lset, t.discoveredLabels)
-	return lset
+	return t.discoveredLabels.Copy()
 }
 
 // SetDiscoveredLabels sets new DiscoveredLabels
@@ -205,9 +203,9 @@ func (t *Target) URL() *url.URL {
 		params[k] = make([]string, len(v))
 		copy(params[k], v)
 	}
-	for _, l := range t.labels {
+	t.labels.Range(func(l labels.Label) {
 		if !strings.HasPrefix(l.Name, model.ParamLabelPrefix) {
-			continue
+			return
 		}
 		ks := l.Name[len(model.ParamLabelPrefix):]
 
@@ -216,7 +214,7 @@ func (t *Target) URL() *url.URL {
 		} else {
 			params[ks] = []string{l.Value}
 		}
-	}
+	})
 
 	return &url.URL{
 		Scheme:   t.labels.Get(model.SchemeLabel),
@@ -374,15 +372,15 @@ func PopulateLabels(lset labels.Labels, cfg *config.ScrapeConfig, noDefaultPort
 		}
 	}
 
-	preRelabelLabels := lb.Labels(nil)
-	lset = relabel.Process(preRelabelLabels, cfg.RelabelConfigs...)
+	preRelabelLabels := lb.Labels(labels.EmptyLabels())
+	lset, keep := relabel.Process(preRelabelLabels, cfg.RelabelConfigs...)
 
 	// Check if the target was dropped.
-	if lset == nil {
-		return nil, preRelabelLabels, nil
+	if !keep {
+		return labels.EmptyLabels(), preRelabelLabels, nil
 	}
 	if v := lset.Get(model.AddressLabel); v == "" {
-		return nil, nil, errors.New("no address")
+		return labels.EmptyLabels(), labels.EmptyLabels(), errors.New("no address")
 	}
 
 	lb = labels.NewBuilder(lset)
@@ -413,7 +411,7 @@ func PopulateLabels(lset labels.Labels, cfg *config.ScrapeConfig, noDefaultPort
 		case "https":
 			addr = addr + ":443"
 		default:
-			return nil, nil, errors.Errorf("invalid scheme: %q", cfg.Scheme)
+			return labels.EmptyLabels(), labels.EmptyLabels(), errors.Errorf("invalid scheme: %q", cfg.Scheme)
 		}
 		lb.Set(model.AddressLabel, addr)
 	}
@@ -434,50 +432,54 @@ func PopulateLabels(lset labels.Labels, cfg *config.ScrapeConfig, noDefaultPort
 	}
 
 	if err := config.CheckTargetAddress(model.LabelValue(addr)); err != nil {
-		return nil, nil, err
+		return labels.EmptyLabels(), labels.EmptyLabels(), err
 	}
 
 	interval := lset.Get(model.ScrapeIntervalLabel)
 	intervalDuration, err := model.ParseDuration(interval)
 	if err != nil {
-		return nil, nil, errors.Errorf("error parsing scrape interval: %v", err)
+		return labels.EmptyLabels(), labels.EmptyLabels(), errors.Errorf("error parsing scrape interval: %v", err)
 	}
 	if time.Duration(intervalDuration) == 0 {
-		return nil, nil, errors.New("scrape interval cannot be 0")
+		return labels.EmptyLabels(), labels.EmptyLabels(), errors.New("scrape interval cannot be 0")
 	}
 
 	timeout := lset.Get(model.ScrapeTimeoutLabel)
 	timeoutDuration, err := model.ParseDuration(timeout)
 	if err != nil {
-		return nil, nil, errors.Errorf("error parsing scrape timeout: %v", err)
+		return labels.EmptyLabels(), labels.EmptyLabels(), errors.Errorf("error parsing scrape timeout: %v", err)
 	}
 	if time.Duration(timeoutDuration) == 0 {
-		return nil, nil, errors.New("scrape timeout cannot be 0")
+		return labels.EmptyLabels(), labels.EmptyLabels(), errors.New("scrape timeout cannot be 0")
 	}
 
 	if timeoutDuration > intervalDuration {
-		return nil, nil, errors.Errorf("scrape timeout cannot be greater than scrape interval (%q > %q)", timeout, interval)
+		return labels.EmptyLabels(), labels.EmptyLabels(), errors.Errorf("scrape timeout cannot be greater than scrape interval (%q > %q)", timeout, interval)
 	}
 
 	// Meta labels are deleted after relabelling. Other internal labels propagate to
 	// the target which decides whether they will be part of their label set.
-	for _, l := range lset {
+	lset.Range(func(l labels.Label) {
 		if strings.HasPrefix(l.Name, model.MetaLabelPrefix) {
 			lb.Del(l.Name)
 		}
-	}
+	})
 
 	// Default the instance label to the target address.
 	if v := lset.Get(model.InstanceLabel); v == "" {
 		lb.Set(model.InstanceLabel, addr)
 	}
 
-	res = lb.Labels(nil)
-	for _, l := range res {
+	res = lb.Labels(labels.EmptyLabels())
+	err = res.Validate(func(l labels.Label) error {
 		// Check label values are valid, drop the target if not.
 		if !model.LabelValue(l.Value).IsValid() {
-			return nil, nil, errors.Errorf("invalid label value for %q: %q", l.Name, l.Value)
+			return errors.Errorf("invalid label value for %q: %q", l.Name, l.Value)
 		}
+		return nil
+	})
+	if err != nil {
+		return labels.EmptyLabels(), labels.EmptyLabels(), err
 	}
 	return res, preRelabelLabels, nil
 }
@@ -501,12 +503,12 @@ func TargetsFromGroup(tg *targetgroup.Group, cfg *config.ScrapeConfig, noDefault
 
 		lset := labels.New(lbls...)
 
-		lbls, origLabels, err := PopulateLabels(lset, cfg, noDefaultPort)
+		lset, origLabels, err := PopulateLabels(lset, cfg, noDefaultPort)
 		if err != nil {
 			failures = append(failures, errors.Wrapf(err, "instance %d in group %s", i, tg))
 		}
-		if lbls != nil || origLabels != nil {
-			targets = append(targets, NewTarget(lbls, origLabels, cfg.Params))
+		if !lset.IsEmpty() || !origLabels.IsEmpty() {
+			targets = append(targets, NewTarget(lset, origLabels, cfg.Params))
 		}
 	}
 	return targets, failures